1. Field of the Invention
The invention relates to aerodynamics of turbine airfoils and, in particular, turbine airfoil trailing edges.
2. Description of Related Art
A typical gas turbine engine of the turbofan type generally includes a forward fan and a booster or low pressure compressor, a middle core engine, and a low pressure turbine which powers the fan and booster or low pressure compressor. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are connected by a high pressure shaft. High pressure air from the high pressure compressor is mixed with fuel in the combustor and ignited to form a very hot high energy gas flow. The gas flow passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas flow leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Marine or industrial gas turbine engines have low pressure turbines which power generators, ship propellers, pumps and other devices while turboprops engines use low pressure turbines to power propellers usually through a gearbox.
The high and low pressure turbines have at least one turbine nozzle including at least one row of circumferentially spaced apart airfoils or vanes radially extending between radially inner and outer bands. The vanes are usually hollow having an outer wall that is cooled with cooling air from the compressor. Hot gases flowing over the cooled turbine vane outer wall produces flow and thermal boundary layers along hot outer surfaces of the vane outer wall and end wall hot surfaces of the inner and outer bands over which the hot gases pass. The high and low pressure turbines also have at least one row of turbine rotor blades including circumferentially spaced apart airfoils extending radially outwardly from turbine blade platforms. High pressure turbine airfoils, including stator vanes and rotor blades, typically require internal convection cooling and external film cooling. These airfoils are typically cast including internal cooling features such as cooling air passages and pins and turbulators. Therefore, the turbine airfoils are usually thicker than the compressor airfoils. The trailing edges of some turbine airfoils are tapered down to about 30 to 50 mills. When the hot gas flows over the turbine airfoils, there is vortex shedding at the trailing edge base due to the pressure gradient caused by the thickness of the trailing edge. This vortex shedding causes undesirable pressure losses which are sometimes referred to as blockage effects. Due to the mechanical and casting constraints, the trailing edge thickness cannot be physically further reduced. It is desirable to be able to reduce or eliminate this undesirable pressure losses due to the vortex shedding for better turbine efficiency.